Bifocal contact lenses have heretofore been made by introducing into a lens blank made of one clear plastic material, some bodies of another clear plastic material having a different index of refraction for certain parts of the lens blank and then fusing the materials together at the boundary by heat. Such a method is illustrated in U.S. Pat. No. 3,697,629, in which a lens blank is first made and then the blank is finished into a contact lens by cutting and polishing to the desired curvature. Several methods of inserting the material of different index of refraction are described.
In the case of any bifocal occular aid, the lens remains in the same position with respect to the user's head, and the eyeball turns to shift the angle of view so as to look through a different portion of the lens. In the case of a contact lens, the same angular rotation of the eyeball, of course, makes a very much smaller shift of the area of the lens that is used. It is still necessary for the eyeball to move relative to the lens, which means there is a slight movement of the cornea along the surface of the lens. If the bifocal lens is not symmetrical about the center of the lens, it then becomes necessary to prevent the lens from rotating about its own axis in the course of blinking action of the eyes. It has been found that the lens stays quite well centered on the pupil of the eye, but that some rotation may occur while the lens appears to remain properly in place.